When several parts are assembled it is often the strength of the joint as such which is decisive to the strength of the finished element. In the context of fibre-reinforced laminates purely mechanical joints such as bolting, riveting, etc., are most often not particularly suitable. Instead gluing is most often applied or, if two fibre-reinforced elements are concerned, optionally joining by impregnation of the one element onto the other. In order to achieve a joint with sufficient strength, it is usually necessary to treat the surface of the areas on the one or both of the elements to be joined with a view to achieving larger surface area and hence increased adhesion.
Several such surface treatments are available, the most simple and probable most common one is to grind the surface in those places where the joining is to take place in order to subsequently glue or in any other way obtain joining. Hereby a rough surface is accomplished with a number of exposed fibres. However, the grinding is very time-consuming and a very work-intensive process, and simultaneously it is difficult and expensive to remove the grinding dust resulting from the process, which dust is undesirable for working environment and health considerations. If the finished composite product is subsequently exposed to humidity, remaining grinding dust may also be a great inconvenience, since the dust may collect to form very hard lumps when absorbing humidity.
Other methods of increasing the surface area and hence the adhesion include sanding, chemical degreasing or use of laser, wherein indentations are formed on the surface of the element due to evaporation of material from the surface. However, sanding of the surface entails the same drawbacks with dust and poor working environment as grinding. The working environment is also an issue in case of chemical degreasing and laser treatment. The methods also share the feature that they all destroy the surface of the laminate to some degree or other where the laminate is treated, which makes it more difficult to ensure the same quality of the laminate outside and within the joining zones. A further drawback of those methods for surface treatment is that they are all relatively work-intensive and hence time-consuming. Most often it is also necessary that the laminate maintains its shape and remains braced during the surface treatment, which is a drawback since it is both space-consuming and prevents the use of the scaffolding for another element, while the finished laminate cures elsewhere. Additionally the joining to the one or the other elements should preferably take place fairly shortly after the surface treatment in order for it to work optimally. When the laser treatment is concerned, it is also an expensive method, both as to acquisition costs of equipment and execution costs.
U.S. Pat. No. 5,968,639 teaches a method of joining several parts, where Z-pinning is used to cause fibres to project from the surface of the one part. During manufacture of the laminate a foam element filled with Z-pins in the desired amount and orientation is arranged on top of the laminate and underneath the vacuum cloth. During the injection the foam material collapses due to the temperature and/or the pressure, whereby the Z-pins are pressed partially down into the laminate and integrated by moulding into its surface. The remainder of the foam material can be removed just before the joining of the laminate to another part and thus serves to protect the Z-pins in the meantime. Here the Z-pins increase the strength of the joint quite considerably, but the manufacturing method is complicated by the need to fairly accurately control the pressure and the temperature during the injection in order to ensure that the foam material collapses to a suitable degree. Moreover the arranging of the Z-pins considerably increases the production time.